To insert a nucleotide into a read in the Sanger Reads Editor, select a nucleotide or a gap in the corresponding location in the Chromatogram Area and switch to the editing mode by pressing Shift+I, selecting Edit–>Insert character/gap in the context menu, or selecting Actions->Edit->Insert character/gap in the main menu. While the cursor is blinking, press a character on the keyboard that you want to insert. After that the editing mode is switched off.
Note that if you press the '-' character on the keyboard while being in the editing mode, a gap is inserted. The other way to insert a gap is by pressing Space, see the Inserting Gap chapter.
You can always cancel the changes by pressing Ctrl+Z.